It is known that in the textile industry every process which produces a thread makes it necessary to store the thread in such a way as to make it available in the most convenient form for the subsequent operations.
One of the most commonly used forms of the storage is provided by what is known as a spool, in other words a cylindrical member onto which the thread is wound to create a bobbin (the spool) which must have closely specified characteristics such as diameter, weight, shape, precision, and speed of unwinding. The characteristics are capable of identifying the greater or lesser suitability of a certain type of spool for the subsequent processing which may require high unwinding speed, the lowest possible unwinding tension, a uniform density or high volume.
It is also known that the above mentioned characteristics are substantially determined by the conditions of winding of the spool. In particular, two types of spool winding are known, designated "precision" and "course", which provide different types of spools. In the first case the spool is driven by the spindle and undergoes a constant number of revolutions in the time interval determined by the outward and return movement of the thread guide, thus keeping the relationship between the angular velocities of the spool and thread guide constant throughout the formation of the spool, although in these conditions the angle of laying--or crossing--of the thread must decrease with the increase of the diameter of the spool, thus causing an increase in the density of the spool which becomes excessively wide and may become unstable. In the case of course winding, on the other hand, the spool is driven indirectly by a driving cylinder which also moves the thread traversing device, in other words the grooved drum which determines the angle of laying of the thread with respect to the spool axis. In the latter when the diameter increases during spooling, given a constant thread advance speed, the relationship between the angular velocity of the spool and that of the thread guide changes, but the angle (B in FIG. 1) of laying--or crossing--of the thread remains constant, thus forming a stable and regular spool of uniform density. Under these conditions, however, since the turns ratio decreases as the spool diameter increases, the probability of superimposition of the thread, in other words, the occurrence of the undesirable phenomenon known as "mirror winding", increases, and consequently a spool is formed which, during unwinding, has characteristics which, at certain moments corresponding to the points of superimposition of the thread, differ considerably from the basic characteristics of the spool.
It is also known from the prior art that numerous attempts have been made to produce equipment capable of controlling the winding characteristics over a period of time to provide spools with the best characteristics of the two types of winding. In particular, methods and corresponding equipment for providing such control are known from the publications DE-OS 26 49 780 and U.S. Pat. No. 3,235,191. However, both publications are based on the control of the rotation speed of the winding cylinders to form windings of the rough type, but with a variation of the crossing angle within restricted limits approximating to a precision winding.
The solutions offered by the above-mentioned reference, however, have the disadvantage of basing the control procedure on the monitoring of the rotation speed, thus introducing an error into the determination of the thread position, given the variable time which relates space to speed.
Given the number of turns required to create a spool, even a small error will tend to increase over a period of time, thus increasing the probability of error in the control and reducing the probability of obtaining a spool with the desired characteristics, leading, for example, to the aforesaid phenomenon of mirror winding.